This invention pertains to, inter alia, X-ray sources, more specifically to X-ray sources useful for any of various X-ray apparatus such as X-ray microscopes, X-ray analysis devices, and X-ray microlithography apparatus. Even more specifically, the invention pertains to X-ray sources that produce X-rays from a plasma produced by a target material highly energized by laser pulses or electrical discharge.
Laser-plasma X-ray sources (hereinafter abbreviated as xe2x80x9cLPXxe2x80x9d sources) produce X-rays from a plasma generated by focusing a pulsed laser light on a target material situated inside a vacuum chamber. The laser light pulses convert the target material into the plasma, from which the X-rays are produced. LPX sources are small but nevertheless generate X-rays having an intensity comparable to the intensity of X-rays produced by undulators. Other small X-ray sources include dense plasma focus (DPF) sources that produce X-rays from an electrically produced discharge plasma. DPF sources also produce large quantities of X-rays, and have a higher conversion efficiency of X-rays to input power, and are lower in cost, than LPX sources.
In LPX and DPF sources, the target material and any other material located in or near the plasma are atomized, ionized, or generally fragmented (the products of such fragmentation are termed herein xe2x80x9cflying debrisxe2x80x9d). The particles of flying debris propagate to neighboring components (e.g., X-ray optical elements) to which the debris adheres and on which the debris accumulates. These deposits diminish the performance (e.g., reflectivity or transmissivity) of the components. Also, collisions of particles of the flying debris with neighboring optical components damage the components.
According to one conventional approach to reducing the problem of flying debris in LPX sources, the target material is a gas at room temperature (e.g., nitrogen, carbon dioxide, krypton, and xenon). The gaseous target material is discharged from a nozzle while a pulsed beam of laser light is being irradiated onto the discharge stream of gas. According to another approach, the discharged target material is configured as a gaseous cluster produced by adiabatic expansion. Because they are gaseous, target materials produced in such manners tend not to accumulate on neighboring optical components. However, the plasma itself produces and emits high-velocity atoms, ions, and electrons that collide with the discharge nozzle and with components near the discharge nozzle. These collisions erode the nozzle and the components, producing flying debris that propagates to surrounding regions and accumulates on neighboring optical components. Consequently, an LPX source that produces no flying debris has yet to be realized.
Meanwhile, to decrease the rate at which flying debris is produced in LPX and DPF sources, efforts have been made to fabricate components of these sources (such as nozzles and electrodes) using materials having high melting points and high hardness, such as tungsten or tantalum. Another approach has been to decrease the operating voltage of the source. Unfortunately, neither approach has resulted in zero flying debris.
In addition, flying debris is not emitted uniformily in all directions. Rather, the particles tend to be emitted preferentially according to a certain asymmetric angular distribution. For instance, in LPX sources that utilize a gas-jet nozzle, fewer particles of flying debris propagate in the gas-discharge direction (i.e., along the gas-discharge axis). The quantity of flying debris increases with increases in the angle from the discharge axis.
For X-ray illumination purposes as exploited in X-ray microlithography apparatus, for example, illumination-optical systems have been proposed that utilize fly-eye mirrors. In this regard, reference is made to FIG. 7(B) depicting a system that receives a collimated beam 702 of X-rays that is reflected successively from two fly-eye mirrors 703, 704 before being reflected by illumination mirrors 705-706 to a reflective reticle 707. From the reticle 707, the X-rays are reflected by a projection-mirror array 708 to a substrate 709. As shown in FIG. 7(A), a typical fly-eye mirror 700 comprises multiple arc-shaped micro-elements grouped together. Each fly-eye mirror, such as that shown in FIG. 7(A), facilitates the achievement of a constant X-ray intensity distribution at the reticle 707. (See Japan Kxc3x4kai Patent Document No. Hei 11-312638). If X-rays incident to a fly-eye mirror have an axially symmetric distribution of X-ray intensity around the center axis of the fly-eye mirror, then the beamlets of reflected X-ray light from the various micro-elements of the fly-eye mirror reinforce each other and make uniform the intensity distribution of X-ray light at the reticle. However, if the X-ray beam incident to the fly-eye mirror is asymmetric around the center axis of the fly-eye mirror, then the fly-eye mirror will not adequately compensate for intensity variations of the incident beam. Consequently, the intensity distribution of the X-ray beam reflected from the fly-eye mirror will not be uniform at the reticle.
The angular distribution of X-rays radiated from a gas-nozzle LPX generally is rotationally symmetric around the gas-discharge axis. If a paraboloidal mirror (i.e., a mirror having a reflective surface configured as a paraboloid of revolution) were situated such that its axis of revolution is coincident with the gas-discharge axis, then X-rays reflected by the paraboloidal mirror should be a collimated beam having an intensity distribution nearly symmetrical to the gas-discharge axis. Thus, an X-ray flux suitable for the illumination-optical system described above could be formed. However, the angular distribution, relative to the gas-discharge axis, of emitted flying debris typically is not symmetrical. Rather, the angular distribution of the flying debris depends upon the plasma producing the debris and on the position of the nozzle (in the case of a gas-discharge LPX source) or the electrode (in the case of a discharge-plasma DPF source).
As a result of the phenomena summarized above, operation of an X-ray source for a long period of time is accompanied by a progressively more asymmetric distribution of X-ray intensity produced by the source, due to the axially asymmetric accumulation of flying debris on neighboring optical components. With respect to use of such a source in an X-ray microlithography apparatus, this asymmetric distribution of X-rays results in variations in the axial distribution of X-rays illuminating a reticle, with corresponding inaccuracies in the transfer of a reticle pattern to a substrate.
In view of the shortcomings of conventional apparatus and methods as summarized above, an object of the invention is to make any deposits of flying debris on an X-ray optical component situated adjacent the X-ray source rotationally symmetric about a propagation axis of the X-rays. Thus, the intensity distribution of the X-ray flux from the source is maintained rotationally symmetric, even in situations in which the X-ray source exhibits an asymmetrical distribution of emissions of flying debris.
Another object is to provide X-ray optical systems, situated adjacent the X-ray source, configured to rotate one or more neighboring optical components about the optical axis (propagation axis) of the X-ray beam. As a result, in the context of X-ray microlithography for example, the intensity distribution of the X-ray beam at the reticle remains uniform about the optical axis. In an X-ray microlithography apparatus, this axial uniformity of the beam allows the reticle pattern to be transferred accurately to the substrate.
To such ends, and according to a first aspect of the invention, X-ray sources are provided that generate X-rays from a plasma produced by directing pulsed laser light onto a target material in a vacuum chamber evacuated to a subatmospheric pressure. An embodiment of such a source includes a device for directing an X-ray flux from the plasma to a downstream optical system. The device comprises an optical element contained in the vacuum chamber and situated such that X-rays from the plasma are incident on the optical element. The optical element has an axis of rotational symmetry and is configured to direct the X-ray flux to the downstream optical system. The device also comprises a rotational actuator situated relative to the optical element and configured to rotate the optical element about the axis.
By rotating the optical element, the distribution of any deposited flying debris on the optical element is rotationally symmetric. As a result, for example, even if the angular distribution of produced flying debris is asymmetric, the intensity distribution of the X-rays propagating from the optical element is axially symmetric.
In this embodiment, the optical element can be, for example, an X-ray reflective mirror. The X-ray reflective mirror can be, for example, a multi-layer mirror, a grazing-incidence mirror, a spherical mirror, a paraboloidal mirror, a planar mirror, an ellipsoidal mirror, or an a spherical mirror. Stated another way, the mirror can comprise a reflective surface having, for example, any of the following profiles: spherical, paraboloidal, planar, ellipsoidal, or a spherical, or any combination of these profiles. Alternatively, the optical element can be an optical filter or a diffractive element.
This embodiment can include a position detector, a controller, and a positional actuator. The position detector is situated and configured to detect a position of the optical element, and is connected to the controller. The positional actuator, to which the optical element is mounted, also is connected to the controller. The positional actuator is configured, when commanded by the controller, to move the optical element as required for maintaining a desired position of the optical element, based on a signal from the position detector. The positional actuator can comprise an X-direction linear stage, a Y-direction linear stage, and a Z-direction linear stage. It also can include a device for tilting the optical element to realign the axis of rotational symmetry of the element with another axis, such as the propagation axis of the X-rays propagating from the element. The position detector can have any of several possible configurations, such as a contact-needle displacement gauge or a device employing a laser and a light receiver (e.g., photodiode). In the latter instance, the laser is directed at the optical element and the light receiver is oriented so as to receive laser light reflected from the optical element. In any event, with an X-ray source including these features, any variation of the orientation of the optical axis of the X-ray flux can be maintained within specified tolerances during rotation of the optical element.
According to another aspect of the invention, X-ray sources are provided. An embodiment of such a source comprises a vacuum chamber, an X-ray generator, an optical element, and an actuating device. The X-ray generator is situated within the vacuum chamber and is configured to produce a plasma sufficiently energized so as to produce X-rays. The optical element is contained in the vacuum chamber and is situated such that X-rays from the plasma are incident on the optical element. The optical element has an axis of rotational symmetry and is configured to direct the X-ray flux in a downstream direction (such as to an X-ray optical system). The actuating device is situated relative to the optical element and is configured to rotate the optical element about the axis. The X-ray generator can be, for example, a laser plasma X-ray device or a plasma-discharge X-ray device. As noted above, the optical element can be an X-ray reflective mirror or an optical filter (e.g., a filter transmissive to X-rays but not to visible light). This embodiment also can include a position detector, a controller, and a positional actuator as summarized above.
According to another aspect of the invention, X-ray optical systems are provided that include any of the X-ray sources summarized above.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.